1. Field of the Invention
The invention relates generally to the field of electromagnetic survey apparatus for subsurface exploration in the Earth. More particularly, the invention relates to structures for detector electrodes and arrays thereof for detection of induced voltages resulting from electromagnetic fields imparted into the Earth.
2. Background Art
Electromagnetic surveying is used for, among other purposes, determining the presence of hydrocarbon bearing structures in the Earth's subsurface. Electromagnetic surveying includes what are called “controlled source” survey techniques. Controlled source electromagnetic surveying techniques include imparting an electric current or a magnetic field into the Earth, when such surveys are conducted on land, or imparting the same into sediments below the water bottom (sea floor) when such surveys are conducted in a marine environment. The techniques include measuring voltages and/or magnetic fields induced in electrodes, antennas and/or magnetometers disposed at the Earth's surface or on the sea floor. The voltages and/or magnetic fields are induced by interaction of the electromagnetic field caused by the electric current and/or magnetic field imparted into the Earth's subsurface (through the water bottom in marine surveys) with the subsurface Earth formations.
Marine controlled source electromagnetic surveying known in the art typically includes imparting alternating electric current into the sediments below the water bottom by applying current from a source, usually disposed on a survey vessel, to a dipole electrode towed by the survey vessel. A dipole electrode is typically an insulated electrical cable having two electrodes thereon at a selected spacing, sometimes 300 to 1000 meters or more. The alternating current has one or more selected frequencies, typically within a range of about 0.1 to 100 Hz. A plurality of detector electrodes is disposed on the water bottom at spaced apart locations, and the detector electrodes are connected to devices that record the voltages induced across various pairs of such electrodes. Such surveying is known as frequency domain controlled source electromagnetic (f-CSEM) surveying. f-CSEM surveying techniques are described, for example, in Sinha, M. C. Patel, P. D., Unsworth, M. J., Owen, T. R. E., and MacCormack, M. G. R. (1990), An active source electromagnetic sounding system for marine use, Marine Geophysical Research, 12, 29-68. Other publications which describe the physics of and the interpretation of electromagnetic subsurface surveying include: Constable, S. C. and Edwards, R. N. (1991), Electrical exploration methods for the seafloor: Investigation in Geophysics No 3, Electromagnetic methods in applied geophysics, vol. 2, application, part B, 931-966; and Cheesman, S. J., Edwards, R. N., and Chave, A. D. (1987), On the theory of sea-floor conductivity mapping using transient electromagnetic systems: Geophysics, 52, No. 2, 204-217.
Another technique for electromagnetic surveying of subsurface Earth formations known in the art is transient controlled source electromagnetic (t-CSEM) surveying. In t-CSEM surveying, electric current is imparted into the Earth's subsurface using electrodes on a cable similar to those explained above as used for f-CSEM. The electric current may be direct current (DC). At a selected time or times, the electric current is switched off, and induced voltages are measured, typically with respect to time over a selected time interval, using electrodes disposed on the water bottom as previously explained with reference to f-CSEM surveying. Structure and composition of the Earth's subsurface are inferred by the time distribution of the induced voltages. t-CSEM surveying techniques are described, for example, in Strack, K.-M. (1992), Exploration with deep transient electromagnetics, Elsevier, 373 pp. (reprinted 1999).
Irrespective of the technique used, the presence of hydrocarbon bearing structures can be inferred because of resistivity contrast between hydrocarbon bearing structures, which can have electrical resistivities in a range of several ohm-meters to several hundred ohm-meters, and those of the adjacent, non hydrocarbon bearing Earth formations, which may have resistivities in a range of about 0.2 ohm-meters to several ohm-meters.
The foregoing electromagnetic survey techniques can be time consuming and expensive to perform, mainly because the detector electrodes are typically disposed in cables that are deployed on the water bottom. Deploying such detector electrode cables typically includes unspooling them from the survey vessel or another deployment vessel, locating the geodetic position of the electrodes after deployment, and retrieving the cables after the survey is completed. To survey a substantial area of the Earth's subsurface, therefore, requires deployment of a substantial number of such cables and/or repeatedly deploying the cables in different positions along the water bottom. The principal reason that water bottom deployed (stationary) detector cables are used is that the voltages induced across pairs of the electrodes from electromagnetic effects are small enough such that noise that would be induced in the electrodes were they to be moved through the water would make it difficult to measure the voltages induced by electromagnetic effects.
Towing electrodes on cables is known in the art for certain types of marine surveying, particularly as stated above, for imparting an electric field into the formations below the water bottom. Using towed electrodes known in the art for electromagnetically induced voltage detecting, however, is difficult to perform using electrodes known in the art, particularly because towed cables vibrate as they move through the water. This phenomenon as it affects electrodes mounted on a cable was studied early on in relation to submarine receiving antennas. As a result of such study a number of noise sources were identified. See, for example, M. L. Burrows, IEEE Trans. Comm., 22 (1974) 540.
A significant source of noise results from the motion of the electrodes and interconnecting cables within the Earth's geomagnetic field, that is, electromagnetic induction. The motion is excited by pressure fluctuations along the cable as it moves through the water, which makes it start vibrating. For a long cable it can be shown that the motion-induced voltage is proportional to v5/2/f2, where v and f are the towing speed and the signal frequency, respectively. Frequencies used for submarine communication antennas are above 60 Hz, and as a result of frequency dependence of the noise, the resulting noise can be dealt with. However, for frequencies often used for hydrocarbon exploration, which are approximately 0.4-0.8 Hz, induction noise is difficult to deal with. Using a formula developed by Burroughs and disclosed in the foregoing IEEE publication, the noise level would be expected to be on the order of 0.3 μV/Hz1/2m at the frequencies of interest and a towing speed of 5 knots. Such noise level is unacceptably high in relation to the voltages expected to be measured in typical electromagnetic surveying.
Other significant noise sources are electrode noise, water motion noise and thermal noise. Electrode noise arises from the water motion disturbing the electrochemical double layer at the electrode surface. Water motion noise can be associated with induction in the geomagnetic field from water turbulence. Thermal noise will always be present if there are temperature gradients proximate the electrodes.
What is needed is a system for acquiring electromagnetic survey data that can be towed in the water similarly to a seismic streamer system such that the speed and efficiency of acquiring electromagnetic survey data are improved. Such a system should be configured to minimize noise that may be induced in the sensing elements as a result of movement of water past the sensing elements and movement of the sensing elements other than along the direction of towing.